1. Field of the Invention
This invention relates to an overlay mark used in IC processes, and more particularly, to an overlay mark for checking the alignment accuracy between a lower layer and a lithography process for defining an upper layer, a method for forming the same, and the application of the same in alignment accuracy checking.
2. Description of the Related Art
As the linewidth of the IC process continuously gets narrower, the alignment accuracy between a lower layer and an upper layer becomes more and more important. Therefore, an overlay mark is generally formed on a wafer for checking the alignment accuracy, wherein the most popular type of overlay mark is the so-called box-in-box (BIB) overlay mark.
A conventional BIB overlay mark is applied to cases where the lower and the upper layers each is defined by a single photomask, including four trenches in a portion of the lower layer that define a first rectangle, and four photoresist bars on the portion of the lower layer that define a second rectangle and are formed in the lithography process for defining the upper layer and surrounded by the four trenches. The four trenches are defined by a single photomask having thereon a corresponding 4-trench pattern, the four photoresist bars are defined by another single photomask having thereon a corresponding 4-bar pattern. By measuring the position of the center of the first rectangle relative to that of the second rectangle, the alignment accuracy between the lower and the upper layers can be derived.
On the other hand, as the process window gets narrower, it is more important to control the critical dimension of the semiconductor device. When an IC process need to form two areas of different pattern arrangements for a layer, two exposure steps constituting a double exposure process are usually performed to the photoresist layer in respective areas to assure that the two areas are defined in predetermined critical dimensions. When at least one of the two patterning process respectively for defining the above lower and upper layers includes two exposure steps using two photomasks, the above-mentioned conventional overlay mark is no longer suitable to use.